The mechanisms behind the alteration of chemical carcinogenesis by the dietary lipotropes, choline, methionine, folic acid and vitamin B12 have been studied. The metabolism and carcinogenic activity of ethionine in different species is being compared. Ornithine decarboxylase, a marker of tumor promotion and a precursor enzyme for the polyamines, is used to study the possible role of methyl insufficiency in tumor promotion. The great drop in growth rates of transformed liver epithelial cells placed in methionine-deficient, homocysteine-supplemented medium is due to their elevated growth rates in complete medium, rather than to an inability to synthesize methionine.